Metal cutting tools have a collet for firmly retaining a tool shank within a collet bore having a bore diameter. The collet is compressible, and a lock nut is employed to apply a compressing force thereon, thereby forcing the collet to grip the tool shank. The collets and lock nuts may allow passage of coolant fluid towards the tool shank, in order to cool down the tool shank or the area of metal being machined by the tool shank. Examples of such cutting tools and lock nuts are disclosed in EP1640091, EP1872888, GB551720, GB2401335, JP2004148429, JP2011016203, U.S. Pat. No. 5,358,360, U.S. Pat. No. 6,746,023, U.S. Pat. No. 6,729,627, U.S. Pat. No. 7,306,238, US2005/0169718, US2009/322042 and US2010/0148455.
It is an object of the present invention to provide an improved lock nut for locking a collet in a cutting tool, while allowing coolant fluid passage between the lock nut and the collet, while the lock nut is suitable for locking collets with different bore diameters.